1. Field of the Invention
The present subject matter relates to an adhesive comprising hyperbranched poly(triazole)s and a method for preparing the same. In particular, the present subject matter relates to a high temperature-resistant metal adhesive comprising hyperbranched poly(triazole)s formed by azide and alkyne monomers by click polymerization.
2. Description of Related Art
Synthesis of 1,2,3-triazoles by 1,3-dipolar cycloaddition of azides and alkynes was discovered by Arthur Michael at the end of the 19th century (1) and significantly developed by Rolf Huisgen in the 1960s (2). This area of research was quiescent until the azide/alkyne click reaction, so called the Sharpless click reaction, was discovered by Sharpless and his co-workers (3). The reaction has many advantages: i) it brings about sufficiently high yielding, ii) it has high tolerance to functional groups, iii) it is insensitive to reaction media, regardless of their protic/aprotic or polar/non-polar character, and iv) it is able to react at various types of interfaces, e.g., solid/liquid, liquid/liquid and solid/solid interfaces (4, 5).
Hyperbranched organometallic polymers were prepared and they are useful as precursors to advanced ceramic materials (U.S. Pat. No. 6,759,502 to Ben Zhong Tang et al.). Synthesizing hyperbranched polymers by the self-condensation of ABn monomers has some limitations, because ABn monomers are usually obtained via tedious synthesis approaches necessary for asymmetric functionality (6, 7). Moreover, ABn monomers are difficult to prepare and purify, and they suffer from self-oligomerization during storage under ambient conditions (7).
A2+B3 type polymerization has been demonstrated to be an alternative route for hyperbranched polymers, as monomers A2 and B3 are easily obtained and are free of self-oligomerization problems encountered in the ABn system. Still, the preparation of hyperbranched polymers requires multistep reactions and tedious product isolation, and long reaction time and poor product solubility are major obstacles (8).
Synthesis of hyperbranched poly(triazole)s by click polymerization was reported by the present inventors (9) where A2+B3 approach was used with easy-to-make and stable-to-keep diazide (A2) and triyne (B3) monomers. The A2/B3 monomers were readily polymerized by a metal-mediated click reaction and thermally catalyzed Huisgen cycloaddition. It was importantly noted that in the absence of a transition-metal catalyst, the reactions were not regioselective, and when catalyzed with Cu- and Ru-catalysts, click polymerization produced hyperbranched polymers with regular 1,4- and 1,5-linkages, respectively. Both polymers are soluble in typical solvents, such as dichloromethane (DCM), tetrahydrofuran (THF), and dimethyl sulfoxide (DMSO), representing the first example of hyperbranched poly(triazole)s with regioregular structures and macroscopic processability. However, adhesive properties of the polymers and their application for adhesives, particularly high temperature-resistant adhesives have not been studied in the report.
Joining metal substrates by adhesive bonding has numerous advantages over other techniques, such as welding, soldering and mechanical fastening, which include reducing weight, improving fatigue resistance, providing uniform stress distribution and potentially reducing cost (10).
Triazoles have long been known for their strong affinity for metal ions and surfaces, which makes them popular components of polymers used as metal coatings and adhesives. The metal adhesive qualities of polymeric 1,2,3-triazoles have been reported by many researchers (11). Polymer structures containing triazoles were found to enhance binding to copper. Adhesive polymers were formed by assembling polyvalent azides and alkynes into crosslinked polymer networks by copper-catalyzed 1,3-dipolar cycloaddition in US 20080311412 to Valery Fokin et al. Sharpless and his co-workers (12, 13) found that Cu(I) species could efficiently catalyze the azide/alkyne click reaction and create a strong affinity with copper surfaces. The increased reaction rate is mostly explained by the promotion of the formation of the Cu(I)-acetylide, reduction in the oxidation of the Cu(I)-species, and prevention of side reactions of the acetylenes (14). Catalysts for phosphorus-containing compounds and sulfur-containing compounds were reported in WO 2009/051025 to Han et al.